[clang] [clang-repl] Allow private type aliases in out-of-line member function return types (PR #178842)

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fogsong233 wrote:

> That makes sense. However, if we do a more invasive refactoring probably we will have to do what I proposed in the previous paragraph.

I’ll try this next. If we strictly reuse the Clang functions, there should be little discernible difference between incremental mode and normal mode.

https://github.com/llvm/llvm-project/pull/178842